Using the methodology of recombinant DNA we plan to look at the chromosomal and structural organization of the specific human gene for ribosomal RNA. The study of human ribosomal DNA is a study of a repetitious DNA, for which there are approximately 200 copies in the cell. The ribosomal DNA exists on at least five nonhomologous human chromosomes. The genes are present in tandem repeat copies on these five chromosomes. The study of human ribosomal DNA permits us to question factors which maintain homogeneity of the gene, and also study factors which permit heterogeneity. Since the genes are on different chromosomes, the study of these genes also permits analysis of the interaction of nonhomologous chromosomes in normal and abnormal inheritance. Human DNA will be restricted by specific restriction nucleases and further separated by gel electrophoresis. DNA will be removed from the gel and that specific segment used to recombine with a suitable and safe lambda vector. Recombinant phages will be selected and plated on E. coli. The phages which show ribosomal RNA hybridization will be isolated, and DNA prepared from them. In this way, purified single genes from the same cell can be compared with other purified single genes from the same cell. Methods for comparison will include heteroduplex formation, melting temperatures, hybrid stabilities, and equilibrium CsCl2. The documentation of the variability between individuals will also be important to establish whether there is selective pressure maintaining a specific genetic sequence. Since only part of the genetic sequence is transcribed, we will be able to compare the selective pressures which are exerted against transcribed sequences as opposed to those which are not transcribed.